1. Field of the Invention
This invention relates to the field of turbomachinery. More particularly, this invention relates to the field of convergent-divergent nozzle structure for creation and expansion of supersonic flow of a Compressible fluid for turbine motive fluid.
2. Description of the Prior Art
Converging-diverging nozzles are needed to create and expand a supersonic stream (at pressure ratios exceeding approximately 1.85) to transform a high energy stream into a high velocity jet with good efficiency and minimal shock, flow separation, or jet deflection.
Supersonic shock fronts and jet deflection at pressure ratios above design level, and expansion-compression shock and flow separation at pressure ratios below design level are particular problems in accomplishing the desired supersonic expansion in turbine machinery, and these problems have several undesirable consequences, including dissipation of available energy, unstable flow conditions, inefficient operation, and imposition of vibratory stress on the associated rotating turbine blade structure leading to damage thereof. The problem of vibratory stress of the blade structure is a particularly serious problem.
Flow separation results in strong, wide wakes at the nozzle exit, resulting in overall poor efficiency, pressure fluctuations and vibratory loading on the rotating turbine blade structure. Exit edge failures of the nozzles have also been encountered.
Jet deflection is a problem particularly associated with pressure ratios above design level, and thus there is a direct relationship between jet deflection and Mach number. Pressure ratio refers, of course, to the ratio of the pressure at the inlet of the nozzle to the pressure in the area into which the nozzle discharges, commonly referred to as the back pressure. The exit angle of the fluid from the nozzles is also related to and affected by the pressure ratio. The exit angle is the acute angle of the axis of the flow stream with the exit plane of the nozzles. A turbine is typically designed to operate at a particular pressure ratio and exit angle; and increases in the pressure ratio will cause a deflection of the nozzle jet in a direction to increase the exit angle. This jet deflection can result in shock waves, expansion waves, and pressure gradients with resultant stresses on the buckets, i.e., the rotating turbine blades. As a result, blade failures can occur, axial thrust on the rotating structure can increase and fluctuate unpredictably, and an overall highly inefficient operation results.
Jet deflection also results in an adverse effect on entry of the jet stream into the bucket passages. The cross-sectional area of the jet stream entering the buckets is a direct function of the nozzle exit angle. Jet deflection from increased pressure ratio operation results in an increase in the cross-sectional area of the jet stream entering a bucket passage, and the buckets will not function properly if the cross-sectional area of the entering stream exceeds a design multiple of the throat area of the bucket passage.
Conventional nozzles for convergent-divergent supersonic expansion are of two general types. One type has convergent-divergent profiles machined into opposed side surfaces of adjacent nozzles so as to define, in one dimension, convergent-divergent passages between these profiled side surfaces. The top and bottom surfaces of such passages are parallel to each other to define a constant height for the convergent-divergent passage or the height may vary linear between nozzle inlet and exit. The other type of conventional nozzle is in the form of round nozzle passages, as used in drilled and reamed nozzle blocks. These nozzle blocks may be circular in cross-section and thus two-dimensionally convergent-divergent. In any case, the divergence and the throat of the passage are not defined by the inner and outer circumferential walls of the passage.
Both of the above discussed general types of convergent-divergent divergent nozzles do, however, encounter the several problems of jet separation, jet deflection and vibratory excitation damage to turbine blades. These standard nozzle structures usually have a very small spacing from the associated rotating blades; usually on the order of about one-sixteenth of an inch. Any shock waves which occur in the expanding fluid stream must either be dissipated in that narrow space or else the rotating airfoils are buffeted. Since most of such shocks cannot be dissipated in that small space, a great deal of such undesirable buffeting does occur.
Conventional convergent-divergent airfoil-type nozzles also experience flow interruptions at the discharge plane because of the physical fact that there must be some spacing ("trailing edges") between adjacent flow paths. This flow interruption also reduces efficiency.
Conventional convergent-divergent nozzle configurations also present a capacity-size problem. In the one dimensional profiled nozzles increased capacity is achieved by increasing the width, i.e., by widening the spacing, between contoured surfaces, thus increasing the dimension tangential with respect to the turbine wheel. In the round nozzles, increased capacity is achieved by enlarging the contoured circular passages, thus also increasing the tangential dimension. Accordingly, increased capacity invariably results in increased circumferential size of the turbomachinery, a result which is often very undesirable, or which may result in excessive tip-speeds.